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Chapter 3: Urban Climate

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Chapter 3: Urban Climate 3 Urban climate Processes, trends, and projections Coordinating Lead Authors: Reginald Blake (New York City, Kingston), Alice Grimm (Curitiba), Toshiaki Ichinose (Tokyo), Radley Horton (New York City) Lead Authors: Stuart Gaffi n (New York City), Shu Jiong (Shanghai), Daniel Bader (New York City), L. DeWayne Cecil (Idaho Falls) This chapter should be cited as: Blake, R., A. Grimm, T. Ichinose, R. Horton, S. Gaffi n, S. Jiong, D. Bader, L. D. Cecil, 2011: Urban climate: Processes, trends, and projections. Climate Change and Cities: First Assessment Report of the Urban Climate Change Research Network, C. Rosenzweig, W. D. Solecki, S. A. Hammer, S. Mehrotra, Eds., Cambridge University Press, Cambridge, UK, 43–81. Climate change and cities ~3.33 billion today to ~6.4 billion in 2050, about a 90 percent 3.1 Introduction increase. These numbers underscore the fact that urban climate is becoming the dominant environment for most of humanity . Cities play a multidimensional role in the climate change story. Urban climate effects, in particular the urban heat island This chapter presents information on four interrelated com- effect, comprise some of the oldest observations in climatology, ponents of urban climate: (i) the urban heat island effect and air dating from the early nineteenth century work of meteorologist pollution, (ii) the current climate and historical climate trends, Luke Howard (Howard, 1820 ). This substantially predates the (iii) the role of natural climate variability, and (iv) climate earliest scientifi c thought about human fossil fuel combustion and change projections due to worldwide greenhouse gas increases. global warming by chemist Svante Arrhenius (Arrhenius, 1896 ). Figure 3.2 provides a schematic of key interactions within the As areas of high population density and economic activity, cities urban climate system. may be responsible for upwards of 40 percent of total world- wide greenhouse gas emissions (Satterthwaite, 2008 ), although Teasing out the relative infl uences of these components on various sources have claimed percentages as high as 80 percent urban climate is challenging. Natural variability can occur at (reviewed in Satterthwaite, 2008). Figure 3.1 shows a remotely multidecadal timescales, comparable to the timescales used for sensed map of nocturnal lighting from urban areas that is vis- historical analysis and to long-term greenhouse gas forcing. ible from space and vividly illustrates one prodigious source of Another challenge is that different climate factors may not be energy use in cities. Megacities, often located on the coasts and independent. For example, climate change may infl uence the often containing vulnerable population s, are also highly suscep- amplitude and periodicity of natural variability, such as the tible to climate change impacts, in particular sea level rise. At intensity and frequency of the El Niño–Southern Oscillation the same time, as centers of economic growth, information, and (ENSO). technological innovation, cities will play a positive role in both climate change adaptation and mitigation strategies. To survey these issues, twelve cities are selected as examples. The twelve focus cities in this chapter – Athens (Greece), Dakar Urban population recently surpassed non-urban population (Senegal), Delhi (India), Harare ( Zimbabwe ), Kingston (Jamaica), worldwide and is projected to grow from 50 percent currently London (UK), Melbourne (Australia), New York City (USA), São to 70 percent by 2050 (UNFPA, 2007 ). The urban population Paulo ( Brazil ), Shanghai ( China ), Tokyo ( Japan ), and Toronto growth rate will be even more rapid in developing countries. ( Canada ) ( Figure 3.3 ) – share a range of characteristics but also In terms of absolute numbers, urban population will grow from differ in key respects. They are all large, dynamic, and vibrant Figure 3.1: View of Earth at night: Areas of light show densely populated, urban areas. Source: NASA 44 Urban climate: Processes, trends, and projections tends to be more diffi cult in cities with limited resources; typi- cally but not always the case in developing countries. Our focus cities include examples from both the developed and developing world, with some having already taken steps to prepare for cli- mate change and others that have yet to start. Table 3.1 gives some summary statistics on socio-economic and geographic data, and mean climate for the focus cities. 3.1.1 Effects of cities on temperature: urban heat islands Urban areas are among the most profoundly altered land- scapes away from natural ecosystems and processes. Figure 3.4 illustrates a recent reconstruction of the verdant “Mannahatta” island circa 1609 as compared to the current landscape 400 years later (Sanderson and Boyer, 2009 ). Pondering this visually arresting reconstruction, it is not surprising that cities have altered microclimates with, among other effects, signifi cantly Figure 3.2: Conceptual framework of this chapter showing major components impacting urban climate. elevated surface and air temperatures. The elevation in temperatures is most generally explained in terms of the basic surface energy balance processes of urban areas, and act as hubs of social and economic activity. They shortwave and longwave radiation exchange, latent, sen- also feature long-term twentieth-century climate records that allow sible, and conductive heat flows (Oke, 1987). With respect trend detection, projections, and impact analysis. All the cities to shortwave, or solar, radiation , surface albedo refers to the selected are likely to experience signifi cant climate change this reflectivity of a surface to visible light and is measured from century, and several illustrate the infl uence of climate variability 0 to 100 percent reflectivity. The regional albedo of cities systems such as ENSO. Some of the city examples demonstrate is significantly lower than natural surfaces due to the pre- unique vulnerabilities to extreme climate events. ponderance of dark asphalt roadways, rooftops, and urban canyon light trapping. These urban features have typical With regard to differences, the selection includes a range for albedo values below 15 percent (Table 2-4 in Rosenzweig et geography and economic development levels. The various geo- al., 2006). This leads to efficient shortwave radiation absorp- graphic locations allow examination of climate change impacts tion. The urban skyline, with deep urban canyons, results in in multiple climate zones. Covering multiple climate zones also a greatly reduced skyview at street level and this impedes allows for the examination of the infl uence of the major natural longwave radiative cooling processes. This urban vertical climate variability systems such as ENSO. geometry further impacts winds, generally reducing ventila- tion and sensible heat cooling. The replacement of natural A range of economic development examples is important for soil and vegetation with impervious surfaces leads to greatly vulnerability studies. Adaptation planning for climate change reduced evapotranspiration and latent heat cooling. The dense Figure 3.3: Map of cities highlighted in this chapter. The cities were selected based on socio-economic factors and the availability of long-term climate data. 45 Climate change and cities Table 3.1: City statistics and mean temperature and precipitation for 1971–2000.∗ City Latitude Longitude Population Mean annual temperature Annual precipitation Athens 37.9 N 23.7 E 789,166 (2001) 17.8 °C 381 mm Dakar 14.7 N 17.5 W 1,075,582 (2007) 24.0 °C 357 mm Delhi 28.6 N 77.2 E 9,879,172 (2001) 25.1 °C 781 mm Harare 17.8 S 31.0 E 1,435,784 (2002) 18.1 °C 830 mm Kingston 17.9 N 76.8 W 579,137 (2001) 27.5 °C 691 mm London 51.3 N 0.4 W 7,556,900 (2007) 9.7 °C 643 mm Melbourne 37.8 S 145.0 E 3,806,092 (2007) 15.7 °C 652 mm New York City 40.8 N 74.0 W 8,274,527 (2007) 12.8 °C 1181 mm São Paulo 23.5 S 46.4 W 11,016,703 (2005) 19.5 °C 1566 mm Shanghai 31.5 N 121.4 E 14,348,535 (2000) 16.4 °C 1155 mm Tokyo 35.7 N 139.8 E 8,489,653 (2005) 16.2 °C 1464 mm Toronto 43.7 N 79.0 W 2,503,281 (2006) 7.5 °C 793 mm ∗Population data for all cities are from the United Nations Statistics Division Demographic Yearbook, United Kingdom National Statistics Offi ce, and Census of Canada. Annual temperature and precipitation statistics are computed for all cities using data from the National Climatic Data Center Global Historical Climatology Network (NCDC GHCNv2), UK Met Offi ce and Hadley Centre, Australian Bureau of Meteorology, and Environment Canada. Figure 3.4: Manhattan-Mannahatta: on right is a reconstruction of Manhattan Island circa 1609 (called “Mannahatta” by the Lenape native Americans), as compared to today, based on historical landscape ecology and map data. Sources: Markley Boyer / The Mannahatta Project / Wildlife Conservation Society and the aerial view of modern Manhattan, Amiaga Photographers 46 Urban climate: Processes, trends, and projections impervious surfaces with high heat capacity create signifi- Balling and Brazel ( 1987 ), Bornstein and LeRoy ( 1990 ), Jau- cant changes in heat storage and release times as compared to regui and Romales ( 1996), Selover ( 1997), Changnon and natural soil and vegetated surfaces. Westcott (2002 ), and Shepherd et al. ( 2002) have shown evi- dence that corroborates the earlier fi ndings of enhanced pre- There are additional atmospheric and heat source processes cipitation due to urbanization. However, the hypothesis has in cities that interact with these energy balances. Aerosols tend been disputed, and even challenged, by other data studies that to reduce the amount of incoming solar radiation reaching show no local effect on precipitation (Tayanc and Toros, 1997) the surface (a net cooling effect), while elevated ambient or even defi cits in precipitation that accompany urbanization urban carbon dioxide levels may further reduce net radiative (Kaufmann et al., 2007 ). cooling.
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